Abstract
A thermistor having a ceramic main body, which contains a ceramic material as the main constituent. The ceramic main body has at least one electrically insulating layer. The electrically insulating layer is arranged within the ceramic main body and contains a main component which has a composition different from the ceramic material.
Claims
1. A thermistor, having a ceramic main body, which contains a ceramic material as the main constituent, wherein the ceramic main body has at least one electrically insulating layer, the electrically insulating layer is arranged within the ceramic main body, the electrically insulating layer contains a main component which has a composition different from the ceramic material, the electrically isolating layer is arranged within the ceramic main body in such a way that the conducting cross-sectional area of the active volume region of the ceramic main body is reduced.
2. The thermistor according to claim 1, having at least two outer contacts, wherein the electrically insulating layer is arranged within the ceramic main body in such a way that the thermistor has a higher R.sub.25 value than a thermistor that is formed without the electrically insulating layer, but is otherwise the same perpendicular to the direction of flow of the electrical current flowing from one outer electrode to the other.
3. The thermistor according to claim 1, wherein the electrically insulating layer lies in a plane and the electrically insulating layer is kept at a distance from one or more outer surfaces of the ceramic main body that intersect the plane.
4. The thermistor according to claim 1, wherein the electrically insulating layer has at least one cutout.
5. The thermistor according to claim 1, wherein the insulating layer consists of at least two sublayers, which lie in a common plane and are separated from one another by a distance.
6. The thermistor according to claim 1, wherein the electrically insulating layer contains a main component which comprises an inorganic, electrically insulating material.
7. The thermistor according to claim 6, wherein the inorganic, electrically insulating material is selected from a set that includes at least varistor ceramics and capacitor ceramics.
8. The thermistor according to claim 1, wherein the electrically insulating layer contains a main component which has a spinel structure.
9. The thermistor according to claim 1, wherein the electrically insulating layer is formed as a cavity within the ceramic main body.
10. The thermistor according to claim 1, wherein the electrically insulating layer has a thickness of up to 5 μm.
11. The thermistor according to claim 1, wherein the ceramic main body has one or more electrically conductive layers, which are arranged within the ceramic main body in such a way that the electrically conductive layers define an active volume region of the ceramic main body and the electrically insulating layer is arranged within the active volume region.
12. The thermistor according to claim 11, wherein at least one of the electrically conductive layers is in direct contact with at least one outer surface of the ceramic main body.
13. The thermistor according to claim 11, wherein the electrically conductive layers contain at least one material which is selected from a set of metals that includes at least silver, palladium and any silver and palladium alloys.
14. A method for producing a thermistor comprising a ceramic main body, wherein the ceramic main body is produced in such a way that a green sheet stack is formed from a multiplicity of green sheets, is subsequently pressed and sintered, a layer suitable for producing an electrically insulating layer being applied to at least one of the green sheets before the forming of the green sheet stack and wherein the electrically isolating layer is arranged within the ceramic main body in such a way that the conducting cross-sectional area of the active volume region of the ceramic main body is reduced.
15. The method according to claim 14, wherein the layer suitable for producing an electrically insulating layer contains an electrically insulating material.
16. The method according to claim 14, wherein the layer suitable for producing an electrically insulating layer, contains a thermally decomposable material which after the sintering forms an electrically insulating cavity.
17. The method according to claim 14, wherein a template is used to produce the electrically insulating layer, the template being configured in such a way that the electrically insulating layer has a high imaging fidelity with respect to the template.
18. The method according to claim 17, wherein a screen printing screen is used as the template, the screen printing screen having a negative of the electrically insulating layer and at least one bar, which divides the negative into at least two separate regions.
19. The method according to claim 14, wherein an electrically conductive layer is applied to at least one further one of the green sheets.
20. The method according to claim 14, wherein the green sheets are stacked along a longitudinal axis of the ceramic main body.
21. The method according to claim 14, wherein at least two outer contacts are applied to the ceramic main body and wherein the electrically insulating layer is arranged perpendicular to the direction of flow of the electrical current flowing from one outer electrode to the other.
Description
[0039] The invention is described in more detail below on the basis of schematic representations of exemplary embodiments.
[0040] FIG. 1 shows a spatial representation of a thermistor.
[0041] FIG. 2 shows an embodiment of a thermistor in cross section and in longitudinal section.
[0042] FIG. 3 shows a further embodiment of a thermistor in cross section and in longitudinal section.
[0043] FIG. 4 shows a further embodiment of a thermistor in longitudinal section and in cross section.
[0044] FIG. 5 shows a further embodiment of a thermistor in longitudinal section and in cross section.
[0045] FIG. 6 shows a further embodiment of a thermistor in longitudinal section and in cross section.
[0046] Elements that are the same, similar or appear to be the same are provided with the same reference signes in the figures. The figures and the relative sizes of elements in the figures are not drawn to scale.
[0047] FIG. 1 shows a thermistor 10, which comprises a ceramic main body 1 and outer contacts (not shown). The spatial extent of the ceramic main body 1 is illustrated by dimensioning arrows x, y and z. Since the dimensioning arrows x, y and z respectively run parallel to an axis of the same name in a Cartesian coordinate system, the dimensioning arrows are also referred to here and hereinafter as the corresponding axis of the coordinate system. In other words, the dimensioning arrow x corresponds to an x axis, the dimensioning arrow y corresponds to a y axis and the dimensioning arrow z corresponds to a z axis in a Cartesian coordinate system. On account of the great similarity of the embodiments described below with the thermistor 10 shown here, the designations of the axes are used analogously for the following figures.
[0048] FIG. 2A shows in longitudinal section an embodiment of a thermistor 10 similar to that described in FIG. 1. The longitudinal section runs through the ceramic main body parallel to a plane which is defined by the x axis and the z axis of the ceramic main body. The ceramic main body 1 has five electrically insulating layers 3. Furthermore, the thermistor 10 has two outer contacts 2. The electrically insulating layers 3 are arranged parallel to one another. Furthermore, the electrically insulating layers 3 are arranged perpendicularly to a longitudinal axis 6 of the ceramic main body 1.
[0049] The longitudinal axis of the ceramic main body runs centrally through the ceramic main body and parallel to the x axis of the ceramic main body, the x axis corresponding to the direction of greatest extent of the ceramic main body 1. The direction of flow of the electrical current runs parallel to the x axis of the ceramic main body 1.
[0050] FIG. 2B shows a cross section of the thermistor 10 described in FIG. 2A. The cross section runs through the ceramic main body parallel to a plane which is defined by the y axis and the z axis of the ceramic main body. The electrically insulating layers 3 have a quadratic cutout 4. The surface area of the electrically insulating layer 3 plus the surface area of the cutout 4 corresponds to an actual cross-sectional area 7 of the ceramic main body. The actual cross-sectional area is exclusively delimited by outer surfaces 9. Furthermore, the electrically insulating layers 3 are formed in such a way that they are in direct contact with the outer surfaces 9 of the ceramic main body 1. The shape and size of the cutout 4 allow a conducting cross-sectional area L of the ceramic main body 1 to be precisely defined. This makes it possible to set the R25 value of the thermistor 10 very exactly. Here, the conducting cross-sectional area L corresponds to the surface area of the cutout 4.
[0051] FIGS. 3A and 3B show in longitudinal section and in cross section a further embodiment of a thermistor 10 similar to that shown in FIGS. 2A and 2B, the electrically insulating layers 3 being kept at a distance from the outer surfaces 9 of the ceramic main body 1. This has the effect of forming a border region 5, which is not covered by the electrically insulating layer 3 and completely encloses the electrically insulating layers 3. The surface area of the cutout 4, the surface area of the electrically insulating layer 3 and the surface area of the edge region 5 together correspond to the actual cross-sectional area 7 of the ceramic main body, which is exclusively delimited by the outer surfaces 9. The conducting cross-sectional area L is defined here by the surface areas of the cutout 4 and of the border region 5. The electrically insulating layers 3 are kept at a distance from the outer surfaces 9 of the ceramic main body 1 as a result of the method for producing the ceramic main body 1. The configuration described here of the electrically insulating layers 3 has the effect that a delamination of green sheets from a green sheet stack that is formed during the production process of the ceramic main body 1 is effectively prevented.
[0052] FIGS. 4A and 4B show in longitudinal section and in cross section a further embodiment of a thermistor 10 similar to that shown in FIGS. 3A and 3B, the electrically insulating layers 3 of the ceramic main body 1 not having a cutout. In the case of this embodiment, the conducting cross-sectional area L is determined by the surface area of the border region 5, which completely encloses the electrically insulating layers 3. The surface areas of the electrically insulating layer 3 and of the border region 5 together correspond to the actual cross-sectional area 7, which is exclusively delimited by the outer surfaces 9. However, the electrically insulating layers 3 do not have to be thin, as indicated in FIG. 4A, but may be significantly wider, and consequently make up a significant portion of the extent of the ceramic main body 1 in the x direction.
[0053] FIGS. 5A and 5B show in longitudinal section and in cross section a further embodiment of a thermistor 10 similar to that shown in FIGS. 3A and 3B, the electrically insulating layers 3 respectively consisting of two sublayers 3′. The sublayers 3′ are separated from one another by a distance d. The distance d is brought about by a screen printing screen having been used for applying the electrically insulating layers 3. The screen printing screen used for applying the electrically insulating layers 3 has a negative of the electrically insulating layer 3 and a bar, which divides the negative into the sublayers 3′. The width of the bar predetermines the distance d. Bending of the screen printing screen during the application of the electrically insulating layers 3 is largely prevented by the bar, whereby a high imaging fidelity of the electrically insulating layers 3 is achieved. The surface area of the cutout 4 and the surface area of the border region 5, which also extends up to the cutout 4 in the distance d, correspond to the conducting cross-sectional area L. The sum of the surface areas of the electrically insulating layer 3, the cutout 4 and the border region 5 corresponds to the actual cross-sectional area 7, which is delimited exclusively by the outer surfaces 9.
[0054] FIGS. 6A and 6B show in longitudinal section and in cross section a further embodiment of a thermistor 10 similar to that described in FIGS. 3A and 3B. In addition to the electrically insulating layers 3, the ceramic main body 1 of the thermistor 10 comprises two electrically conductive layers 8, which contain silver. The cross section in FIG. 6B runs through one of the two electrically conductive layers 8. The electrically conductive layers 8 are configured in such a way that the electrically conductive layers 8 respectively extend over the entire actual cross-sectional area 7 of the ceramic main body. Furthermore, all of the secondary surfaces 8′ of the electrically conductive layers 8 are in direct contact with the outer surfaces 9 of the ceramic main body 1. Also, the electrically conductive layers 8 are configured in such a way that they are in electrically conducting contact with the outer contacts 2 of the ceramic main body 1 by way of the secondary surfaces 8′. The electrically conductive layers 8 and the outer surfaces 9 of the ceramic main body delimit an active volume region V of the ceramic main body 1 within which the electrically insulating layers 3 are located. By their electrically conducting contact with the outer contacts 2, the electrically conductive layers 8 reduce negative effects that occur due to method-related deviations in the configuration of the outer contacts 2. As a result, thermistors with specific characteristic properties can be produced reproducibly.
[0055] The invention is not restricted to the embodiments shown. In particular, the number, the position and the configuration of the electrically insulating layers 3 and the electrically conductive layers 8 may vary.
LIST OF DESIGNATIONS
[0056] 1 Ceramic main body [0057] 2 Outer contacts [0058] 3 Electrically insulating layer [0059] 3′ Sublayer [0060] 4 Cutout [0061] 5 Border region [0062] 6 Longitudinal axis [0063] 7 Actual cross-sectional area [0064] 8 Electrically conductive layer [0065] 8′ Secondary surface [0066] 9 Outer surface [0067] 10 Thermistor [0068] d Distance [0069] L Conducting cross-sectional area [0070] V Active volume region
